1. Field of the Invention
The present invention relates generally to an image projector, and more particularly to an optical system of the image projector capable of providing color calibration, such that the image projector is built-in or carried by a portable terminal.
2. Description of the Related Art
Lamps irradiating white light are generally used as a light source in the conventional image projector. Recently, an independent light sources having at least three colors to emit respective lights have been used more often as light sources for image projectors. Independent light sources can increase a light transmitting efficiency, and also facilitate miniaturization of products using the independent light source, due to developments in manufacturing technologies for these independent light sources.
An image projector may be composed of light sources for generating light having different visible wavelengths, such as a laser diode or a Light Emitting Diode (LED), and a spatial light modulator for irradiating the generated light on respective pixels on a screen, depending upon needs related to use of the image projector. When an image projector uses an independent light source, the color temperature of an image is determined by a spatial light modulator, which modulates the intensity or the phase of the light according to original image signals, duty ratios of the operational frequencies of the independent light sources, and light intensities of the respective colors, where the light intensities include optical efficiencies of the respective light sources. The self-intensity distributions of the respective light sources also affect the color temperature of the image projector.
In a projector having an LED light source, exemplary distributions of the color temperature due to such various factors may be as shown in FIGS. 1 and 2. Here, FIGS. 1 and 2 illustrate the color temperature distributions of the first and second group projectors, respectively. A red LED typically has a brightness range of 19.0˜24.5 lm and a wavelength range of 615˜620 nm, a green LED has a brightness range of 54.0˜70.0 lm and a wavelength range of 525˜535 nm, and a blue LED has a brightness range of 11.0˜14.5 lm and a wavelength range of 455˜465 nm.
Referring to FIGS. 1 and 2, a portion of the International Commission on Illumination (CIE) 1931 color space and a Planckian locus are illustrated, wherein the transverse axis denotes an x-chromaticity value and the vertical axis illustrates a y-chromaticity value. These values can be distorted as the color temperature varies according to the original image signal as well as the intensity of the original light source and the optical efficiency. The x-chromaticity and y-chromaticity values can also vary according to unit settings of the respective projectors.
In FIGS. 1 and 2, “CCT” refers to a Correlated Color Temperature, CCTmix denotes a target CCT, D65 means 65000K, and D93 means 93000K. In FIG. 1, the CCT distribution exists in a range of 6,457˜18,682K, while in FIG. 2, the CCT distribution is in a range of 6,669˜9,504K.
As shown in FIG. 3, an optical system 10 employed in a conventional image projector may include an illuminating optical part 11, a light modulator 13, and an imaging optical part 12. The illuminating optical part 11 includes at least three light sources 110, 111 and 112 having different wavelengths, at least one filter 113, a lens part 114 and a mirror 115. The light modulator includes a Digital Micromirror Display (DMD) panel 13. The imaging optical part 12 includes a prism 120 and a projection lens 121.
The light sources 110, 111 and 112 include Red-Green-Blue (RGB) LEDs, each generating red light, green light and blue light. Hence, the optical system 10 mixes the red, green, and blue lights to create light having a desired color.
The filter 113 is a dichroic filter used for a color separation and/or synthesis of the light sources 110, 111 and 112, and transmits only a selected wavelength from a visible spectrum, while reflecting the remaining wavelengths.
Meanwhile, the lens 114 includes a fly-eye lens or another similar lens, which enables the brightness of each colored light beam to be uniformly projected onto a screen (not shown), such that the light passing the lens 114 enters the mirror 115. Most of the light beam reflected by the mirror 115 is directed toward the prism 120, while a portion of the light is received at a color sensor (not shown) and given a calibrated value by the color sensor. The above-described conventional projector utilizes a light beam in calibration by taping the light beam at the mirror 115 of the illuminating optical part 11 before the light beam reaches the DMD panel 13 described below.
The prism 120 is a Total Internal Reflection (TIB) prism used in converting a direction of the light beam with a reflecting surface. The prism 120 has a light path that reflects the incident light beam from the mirror 115 toward the DMD panel 13, and transmits the light beam received at the DMD panel 13 toward the projection lens 121 of the imaging optical part 12.
However, in the above-described optical system, since the optical system uses the light beam in the color calibration with the color sensor by taping the light beam before the light beam reaches the spatial light modulator like the DMD panel, the optical system cannot ensure that the color temperature and the color coordinate of the image to be formed on the screen by the spatial light modulator have constant correlations with the color temperature and the color coordinate of the light beam that is taped with respect to the respective projector modules and the image formed on the screen. Constant correlations cannot be maintained, because the respective light sources have wavelength distributions in several nm, and the light paths of the respective projector do not coincide exactly.
If the taping rate is increased in order to address the problem of the varying correlations, the amount of the light that reaches the screen is reduced.